In the circuit are there often exists the desire to convert digital signals to analog signals. Recently, for example, analog curcuits have been substantially replaced by digital circuits. In this case, it is necessary to change back to analog values at the points of intersection with other circuits which cannot be realized digitally. Thus a digital/analog converter is required.
An example for this exists in radio receivers when selected tuning frequencies for various stations have been stored as binary numbers in a memory (e.g. a semiconductor memory) so that they can be selected for tuning and must then be converted to a direct voltage to be applied to the variable capacitance diodes.
In a circuit disclosed in the periodical "Funk-Technik" [Radio Art], 1975, Issue No. 7. pages 180-184, a direct voltage is produced by feeding the rectangular voltage generated by an oscillator in the form of periodic pulses to a lowpass filter which acts as an integrating member. An electronic switch is provided which switches a battery voltage via a resistor to the lowpass filter in the rhythm of the pulse repetition frequency. Due to the integrating effect of the lowpass filter, the time average of the pulses is filtered out and the analog value, i.e., the value of the direct voltage at the output of the lowpass filter, depends upon the keying ratio. Thus a desired direct voltage value is produced by the selection of the keying ratio which is the ratio of the quotient of the pulse duration to the period duration.
It has been found, however, that the known circuits are often insufficient as will be explained below. As already expressed, a digital/analog converter includes an electronic switch which switches a battery voltage U.sub.B via a resistor to an integrating circuit in the rhythm of the pulse repetition frequency f.sub.I. If the switch-off period of the switch is called .tau. and the period duration T, the direct voltage present at the output of the integrating circuit U.sub.I = U.sub.B .multidot. .tau./T. The direct voltage U.sub.I has an alternating voltage U.sub.R superposed on it, which is also called the ripple voltage. This ripple voltage corresponds to the pulse repetition frequency f.sub.I = 1/T. If U.sub.R is to be small, the time constant of the integrating circuit must be large compared to the pulse repetition frequency, i.e., the limit frequency of the lowpass filter forming the integrating circuit must be much lower than the pulse repetition frequency f.sub.I. However -- as will be explained below -- since the latter must not be selected too high, the limit frequency must be relatively low, i.e., the lowpass filter must be relatively narrowbanded. This has the result that upon a change of the keying ratio .tau./T, a relatively long time occurs until the voltage U.sub.I has reached its new stationary value since the time constant in a narrowbanded lowpass is relatively great.
This will be explained with an example. In this example it is assumed that direct voltages U.sub.I can be set within a direct voltage range of 0.5 to 29.5 V in desired steps of 3 mV, i.e., 0.500 - 0.503 - 0.506 V etc., by a change in the keying ratio .tau./T. The steps of 3mV determine the step width .DELTA.U.sub.I. The battery voltage U.sub.B is assumed to be 30 V. Under the above conditions there results the keying ratio .tau./T
of 0.0166 for U.sub.I = 0.5 V and PA1 0.9833 for U.sub.I = 29.5 V
For the step width of .DELTA.U.sub.I = 3 mV the corresponding change in keying ratio is .DELTA..tau./T = 1 .multidot. 10.sup.-4.
If .DELTA..tau. is selected to be 5 .mu.sec, which corresponds to a switching frequency of 200kHz, the period duration T = 50 msec, corresponding to a pulse repetition frequency of 20Hz. Thus a period duration T = 50 msec contains 10,000 of these 5 .mu.sec intervals so that 10,000 direct voltages can be generated which differ by 3 mV. The smallest change (e.g. increase) in a direct voltage U.sub.I by .DELTA.U.sub.I = 3 mV is effected in that the periodic pulses which produce the direct voltage U.sub.I in question are lengthened by .DELTA..tau. = 5 .mu.sec in their time duration. If in this case the ripple voltage U.sub.R is to be about 3 mVpp (from peak to peak) this requires, for a five stage RC integrating circuit, a time constant for the integrating circuit which results in a stationary value of the direct output voltage U.sub.I after about 5 seconds. This long transient period is a drawback, however, particularly during a change from a first to a second desired direct voltage. In the known circuit a relatively long time is required for a change in the keying ratio .tau./T until the direct output voltage has reached its stationary value.
It could be attempted to increase the pulse repetition frequency, and thus shorten the period duration T, so that a lowpass filter with a higher limit frequency, i.e., shorter transient time, could be used. However, there are limits to the increase in frequency in the known circuit since the accuracy of the analog voltage and the required large number of direct voltages values which differ by .DELTA.U.sub.I require a relatively long period duration T and therefore do not permit a decrease in T to eliminate the above-mentioned drawbacks.
It must also be considered that with a short period duration T the above-mentioned minimum time interval .DELTA..tau. would have to be shortened which would result, under certain circumstances, in a technically set minimum value for .DELTA..tau.. On the other hand it is not possible to shorten T if .DELTA..tau. remains constant since in that case the desired smallest value for .DELTA.U.sub.I for the direct voltage could no longer be attained.
High accuracy in the direct voltage, i.e., a low ripple voltage U.sub.R, is required, for example, if the voltage is to serve as the tuning voltage for a radio or television receiver. In order to be able to keep the ripple of the direct voltage at the output of the lowpass filter within acceptable limits, this requires, in the known circuit, expensive and complicated lowpass filters whose limit frequency is relatively low and whose filter edges are very steep with a disadvantageously long transient time.